(48a) Degradation of Lignocellulose and Production of Chemicals Via Sequential Hydrothermal Liquefaction

Conventional hydrothermal conversion of lignocellulose produces low-quality bio-crude at high temperature and pressure. Huge energy loss occurs resulting from polysaccharides conversion to fuels due to the low energy density of these sugars. Especially, lignin decomposition and conversion to fuels and chemicals has been extensively investigated, however, the feedstocks are most restricted to isolated lignin and model lignin compounds. Additionally, distinct lignin isolation methods lead to diverse results owing to the changes of lignin structure during isolation process. The lack of practical depolymerization processes that can produce a limited number of lignin monomers at high yields has limited the options for upgrading lignin. Currently, few studies cover technology that directly convert raw lignocellulose to chemicals to with organic solvent aid process in absence of metal catalyst. Our newly proposed sequential two-stage hydrothermal liquefaction (SeqHTL) process benefits from (1) no pretreatment or pre-isolation step; (2) simultaneous production of bioactive compounds, C5-C6 sugars and their derivatives, and lignin monomers; (3) no toxic organic solvent added.

The preliminary tests showed that two-stage HTL achieved biomass degradation rate of 57.8% at 240 ℃ and 76.6% at 280 ℃, while one stage HTL was 50.0% and 71.4%, respectively. A systematical study has been carried out for SeqHTL: (1) doubling reaction time (from 30 min to 60 min) for both stages increased the solid degradation rate from 57.8% to 70.2%; (2) increasing the biomass load from 1:40 to 1:10 (biomass:water mass ration) slightly decreased the biomass decomposition rate from 57.8% to 52.8%. Further increase on biomass concentration to 1:5 dramatically reduced the decomposition rate to 42.7%; (3) If only increasing second stage temperature by 40 ℃ and keeping first stage unchanged, a 73.4% decomposition rate was achieved. If only increasing second stage reaction time by 30 min and keeping first stage unchanged, a 68.2% decomposition rate was achieved. So, in terms of decomposition rate, the second step is of significance.

Besides pure water system, SeqHTL has also been tested in water-ethanol and acid (H3PO4)-salt (KH2PO4) system. No significant improvement was observed by addition of 50% ethanol at 240℃, however, a tremendous increase in decomposition rate was relarized from 76.6% to 93.0% by addition of 50% ethanol at 280℃. For acid-salt system, the situation turned inverse. At 240℃, acid-aid SeqHTL improved the decomposition rate from 57.8% to 68.2%; while at 280℃, it only slight enhanced the rate from 76.6% to 78.8%. As for solvent concentrations, the decomposition rate keeps increase gradually first and drops quickly after 50 vol% ethanol concentration at 280℃. A maximum rate was obtained at an intermediate concentration, which infers the opposite roles of ethanol in HTL reaction. For acid-salt system, a reverse trend also observed. At 240℃, 100 wt% acid, 50% acid/50% salt, and 100% salt gave rates at 68.2%, 67.6%, and 72.4, respectively. At 280℃, 100 wt% acid, 50% acid/50% salt, and 100% salt gave rates at 78.8%, 72.4%, and 74.8%, respectively. For both cases, the combination of acid and salt showed the lowest degradation ability. The difference is pure salt favored degradation at low temperature while pure acid favored at high temperature. In addition, any acid-salt system at low temperature showed positive degradation ability, i.e. higher than pure water system, while at high temperature, pure salt and 50% acid/50% salt gave negative degradation ability.

Primary products composition analysis supported that first stage harvested mainly C5 sugar degradation products, while second stage enabled mainly C6 sugar degradation products and all lignin derivatives in aqueous phase. An in-depth analytic study is being conducted and a comprehensive SeqHTL pathways will be proposed sooner.